Visit www.amazon.com and you can't miss it (at least in the US - as I understand it, it's not available elsewhere yet).

Other than the rather stiff (US$400) price, this looks like a very interesting device; pretty much what I've been hoping someone might develop in an ebook reader.

The business model seems workable as well. Content seems protected enough that publishers will go along with it, but it's easily accessible to the user. Prices are lower than the equivalent paper book ... seems only fair, but this seems to be a piece of the puzzle that's escaped other ebook attempts.